1. Select the first non-empty Resent-Sender header in the message. If no such header is found, continue with step 2. If it is preceded by a non-empty Resent-From header and one or more Received or Return-Path headers occur after said Resent-From header and before the Resent-Sender header, continue with step 2. Otherwise, proceed to step 5.
2. Select the first non-empty Resent-From header in the message. If a Resent-From header is found, proceed to step 5. Otherwise, continue with step 3.
3. Select all the non-empty Sender headers in the message. If there are no such headers, continue with step 4. If there is exactly one such header, proceed to step 5. If there is more than one such header, proceed to step 6.
4. Select all the non-empty From headers in the message. If there is exactly one such header, continue with step 5. Otherwise, proceed to step 6.
5. A previous step has selected a single header from the message. If that header is malformed (e.g. it appears to contain multiple mailboxes, or the single mailbox is hopelessly malformed, or the single mailbox does not contain a domain name), continue with step 6. Otherwise, return that single mailbox as the Purported Responsible
Address.
6. The message is ill-formed, and it is impossible to determine a Purported Responsible Address.